It has to do with the relationship of GCWR (gross combined weight rating, which is the total weight of truck and trailer combo) and GVWR (gross vehicle weight rating, which is the max weight that should ever be on the vehicles axles).
If GVWR + tow rating = GCWR, then your tow rating is not reduced when you load up the truck. However, the Ridgeline does not fit that equation. From Honda.com:
GVWR=6050 lbs
GCWR=10085 lbs
Tow Rating=5000 lbs
The thing to remember is that the GCWR and GVWR must never be exceeded, so the tow rating is not a real "rating", since fully loading the truck means that your max trailer weight, minus tongue weight, will only be 4035 lbs. Also keep in mind that you need to add the trailer's tongue weight to the tow vehicle's weight when determining if the GVWR is exceeded.
Also understand that fuel, people, accesories (hitch, running boards, heavier tires/wheels, tools, etc.) are all added to the "curb weight" or unloaded weight.